Couldn't be that they are just strechting muscles after a period of
immobility, just like cats and dogs do when they wake up?

I actually think of it that way, ever since we started raising
chickens in the backyard for eggs and meat (none have met the latter
fate yet however), and I see them basking in the sun, lying on one
side, with the wing unfurled. Any fellow chicken owners observed this?

Raymond Thaddeus C. Ancog
Mines and Geosciences Bureau/
Department of Physical Sciences and Mathematics
University of the Philippines, Manila